The American Academy of Pediatrics (AAP) Policy Statement
RE0048 of June 2001 (http://www.aap.org/policy/re0048.html)
states that weight training can be used by pre-adolescents to improve
strength, performance, rehabilitate injuries, and prevent injures. The AAP
recommends that pre-adolescents
Use low-resistance, i.e. low weight, exercises.
Emphasize proper technique, indeed start with no weights
Increase weight gradually.
Workout at least 20 minutes
Minimum of two times per week
If the child's parents do not know how to guide the child in use of resistance equipment they should get instruction
from a trainer or a sensei.